Horses are large running mammals, typically weighing 450-500 kg (990-1100 lbs), and sometimes much more. They are capable of rapid acceleration and attaining speeds of 20 ms-1 (44 mph). Evolution and careful breeding have left horses, particularly horses bred for racing and other athletic contests (as opposed, for example, to draft horses) with comparatively slender and fragile legs comprised substantially of long bones articulated by several series of compact muscles, tendons, and ligaments. The latter three soft tissue structures (as opposed to bone and cartilage, that is) are principally responsible for enabling locomotion either by providing propulsive forces (e.g., upper hind limb musculature), by storing energy (e.g., in the superficial digital flexor tendon (“SDFT”), by abating vibration (e.g., upper forelimb muscles), and by one or more further mechanisms.
The horse's large body size, slender limb structure and occasional need for high speed or rapid acceleration expose the lower limbs, in particular, to risk of injury, either from a single traumatic event such as blunt force trauma or a mis-step of the hoof, or from accumulated micro-damage sustained, for example, during repeated loading of the limb during race training. This can include exposure to too many cycles (frequency) or cycles of excess magnitude (force).
Injury arising from a single incident can affect any of the limb's constituent structures, although the more distal (lower limb) components are generally at greater risk by virtue of their proximity to the ground and ground obstructions. Lower limb structures are placed at greater risk by the paucity of enveloping muscle which, higher up the limb, serves as a ‘fleshy’ buffer to external trauma. As one progresses toward the foot, the limb is increasingly composed solely of bone and adjacent tendon and ligament fibers covered by skin.
Injury resulting from accumulated micro-damage also has preferred sites of incidence. For example, the dorsal (front) surface of the horse's third metacarpal (cannon) bone or the mid-metacarpal region of the SDFT are locations frequently affected in racing thoroughbreds. In particular, the fetlock joint, at which the cannon bone meets the pastern bone, is extremely vulnerable to injury, often with catastrophic results. The device of the preferred embodiment of the present invention (although as noted the invention is not limited thereto) is directed to reduction of the likelihood of injury to the fetlock, as well as to related anatomical structures that are not part of the fetlock per se, such as the superficial digital flexor tendon and the proximal suspensory ligament, as well as to support of the fetlock during rehabilitation after injury or surgery.
As will be appreciated by those of skill in the art, the corresponding joints in fore and rear legs, and the related structures, are called by the same names as a matter of lay use. In scientific terminology, these names change between fore and hind limb. For example, the fetlock of the forelimb, the metacarpo-phalangeal joint, becomes the metatarso-phalangeal joint in the rear limb. The lay terminology is used herein for simplicity. Again, it will be appreciated that the invention is not thus limited.
It should also be noted that the stiffness of biological, soft tissue structures, including tendons and ligaments, increases at very high rates of deformation. Thus, if the fetlock flexor tendons are stretched very quickly (e.g. due to a misstep or fatigue), they can develop much higher resistive loads than if stretched more slowly, even if the joint is not hyperextended per se. This higher load may lead to injury, especially if it occurs repetitively.
Once sustained, injury—be it to bone or soft tissue—requires substantial periods of complete rest or much reduced exercise before the animal can return to normal activity, and in some cases the recovery is never complete. Man's competitive use of horses—which frequently exceeds ‘normal activity’—places additional and frequently unreasonable demands on the healing tissues. As a result, the healing process can be exacerbated and the injury will fail to fully resolve, causing a chronic and sometimes life-long limitation of use. Additionally, while bone is unusual in being able to completely heal itself, soft tissues generally heal with some degree of scar formation which results in added compromise of ambulatory ability, mediated, for example, by pain or adhesions. Scar tissue (unspecialized fibrous tissue in an orientation that is mechanically inferior and/or predisposed to forming adhesions to adjacent structures) is also invariably less strong than undamaged tissue, placing the injured tissue(s) at risk of re-injury.
Recognizing the substantial cost of limb injury to the animal (distress, reduced ambulation, risk of re-injury, etc.) and society (lost use, veterinary bills, investment loss, etc.), researchers have long sought means for reducing the incidence of lower limb injury. Many approaches have been taken including but by no means limited to alteration of ground surface, modification of training techniques, and use of drugs and nutraceuticals. Others have sought to ameliorate the demands placed on the horse's locomotor system during competition by reducing the severity of competitive courses and easing schedules.
Yet another approach has clinicians and researchers attempting to positively impact lower limb biomechanics by limiting extremes of motion, so as to protect both soft and hard tissue structures from being overstressed. The situation is complicated by an incomplete understanding of lower limb mechanics, sometimes resulting in contradictory data findings or theorems. The situation is further exacerbated by the extreme forces occurring within the lower limb during competitive activity—forces which have so far largely precluded the art from preventing extremes of limb motion, for example, by placing the lower limb within protective bandages or boots.
Referring more specifically to the prior art U.S. patents and applications known to the inventors that are directed to protection of the fetlock, and in related fields, Lewis U.S. Pat. No. 121,880 shows a “Stocking for Horses” that is made of rubber and features stiffening ribs to prevent the stocking from working downwardly as the horse moves.
Hyman U.S. Pat. No. 3,209,517 shows a leg support for horses made of closed-cell foam and secured by Velcro straps.
Pomeranz U.S. Pat. No. 4,471,538 is broadly directed to shock-absorbing devices (not specifically for equine applications) employing “rheoprexic fluid” which appears to generically describe a component of a composite “dilatant” material that is used in some embodiments of the invention.
Boyd U.S. Pat. No. 5,107,827 discloses a protective bandage for the fetlocks of horses that is made of Neoprene synthetic rubber, cut out in a complicated fashion and provided with numerous Velcro strips so that the bandage is secured together at numerous points as it is wrapped around the horse's leg.
Hayes et al U.S. Pat. Nos. 5,545,128 and 5,599,290 disclose methods and garments for reducing bone injury due to impact by provision of “shear thickening”, i.e., dilatant, material “in a manner to permit the shunting of impact energy away from the vulnerable [bone] region to the soft tissue region” (claim 1 of the '128 patent).
Walters et al U.S. Pat. Nos. 5,861,175 and 6,368,613 and application 2002/0077368 disclose a method for treatment of articular disorders by injection of fluorocarbons to replace lost synovial fluid.
Chambers U.S. Pat. No. 6,883,466 discloses an animal leg wrap comprising a soft, resilient filler material.
Springs U.S. Pat. No. 6,918,236 shows a breathable equine leg wrap of specific construction. Of interest is the use of phase-change materials for heat removal.
Allen U.S. Pat. Nos. 7,402,147 and 7,789,844 show body limb movement limiters involving a tether paid off a reel, the movement of which is limited by a dilatant fluid.
Greenwald et al U.S. Pat. No. 7,837,640 discloses a joint protective device including an engineered textile including fibers that slide freely over one another at low loads but with increased friction at higher loads, so that the device provides increased resistance to motion at higher loads. The device is also to comprise a “strain rate dependent damping material, so that stiffness in the engineered textile is a non-linear function of displacement, velocity or acceleration”. See claim 1. This material can be one exhibiting “dilatant non-Newtonian behavior such that material stiffness increases with strain rate”—see col. 6, lines 40-42.
Bettin et al U.S. Pat. No. 7,896,019 shows control of the viscosity of a dilatant fluid by application of oscillatory stress, e.g., by way of a piezoelectric transducer, so as to tune the material's characteristics to the application.
Clement patent application 2004/0055543 shows a protective device for a horse's leg that comprises a rigid casing and a padded lining.
White patent application 2006/0107909 shows a tendon and ligament support for the legs of a horse that comprises a gel layer, a dry flex layer, and a Lycra outer layer.
Lindley patent application 2006/0231045 shows a horse leg protector comprises an impact-absorbing inner layer of rubber or foam and a rigid outer housing. Ventilating passages are provided throughout.
Heid et al patent applications 2009/0094949 and 2009/0288377 show equine support boots including sling straps providing support to the fetlock.
Farrow et al patent application 2010/0056973 shows a therapeutic compression device to fit around a limb of a patient.
Green et al patent application 2010/0132099 shows “energy absorbing blends” where a dilatant fluid is entrapped in a solid matrix of a polymer material. It appears possible that this application is directed to a material known to the art as “d3o”. This material is employed in some of the preferred embodiments of the invention, as discussed in detail below.
Husain patent application 2010/0192290 shows a neck protection collar.
Lutz patent application 2011/0034848 shows a compression bandage for horses involving specific closures.
Eggeman U.S. Pat. No. 2,512,925 shows a skid boot for horses, designed to protect the fetlock from contact.
Dever U.S. Pat. No. 2,937,487 shows a protective leg sheath for horses.
Schubert U.S. Pat. No. 3,193,984 shows an inflatable leg sheath for horses.
Porner U.S. Pat. No. 4,099,269 shows a leg sheath for horses with air pockets built into it for impact resistance.
Shapiro U.S. Pat. No. 4,538,602 shows a spirally-wrapped leg protector for horses.
Scott U.S. Pat. No. 5,115,627 shows a horse boot made up of several specified materials.
Gnegy U.S. Pat. No. 5,152,285 shows a horse boot with pockets for insertion of hot or cold packs to treat the horse's leg.
Amato U.S. Pat. No. 5,363,632 shows a boot with an inflatable bladder to support the underside of the fetlock.
Vogt U.S. Pat. No. 5,579,627 shows a support wrap for a horse's leg, including a fetlock-supporting sling strap. Vogt U.S. Pat. No. 5,816,032 is a continuation-in-part of the above and claims a tendon support member.
Wilson U.S. Pat. No. 5,910,126 shows a support wrap for a horse's leg.
Farley U.S. Pat. No. 5,441,015 discloses a method for treatment of an injured horse's leg involving a split rigid cast-type device.
Daly U.S. Pat. No. 7,559,910 discloses a device for preventing over-articulation of the fetlock including an articulated joint including a “pivot arrangement”. Daly teaches both non-extensible tension members for limiting the range of motion of the joint, which are provided with adjustment to allow different limits on the range of motion, as well as resilient members compressed when the joint is flexed, which would help support the tendons. Daly also suggests that friction could be built into the pivot arrangement.
Rogers U.S. Pat. No. 6,151,873 shows a legging for horses including fly netting.
Bard U.S. Pat. No. 6,553,994 shows an orthopedic support molded so as to provide ventilation channels and passages.
Finally, Detty U.S. Pat. No. 5,871,458 shows an equine ankle brace including a cup-like member for fitting over the fetlock.
As will be apparent, most of the prior art devices shown in the patents and applications mentioned above are simply intended to protect the horse's legs from direct impact damage, which, while doubtless beneficial, is insufficient to protect against damage due to repetitive loading, overexertion, hyperextension of the joint, and the like. These damage mechanisms are discussed more fully below. Of the art discussed above, only the device shown in the Daly patent is explicitly intended to prevent hyperextension by mechanical means.
More specifically, it is an object of the present invention to provide a device that provides actual mechanical support to the fetlock, in essence providing additional support to the articular interface, joint capsule, tendons, ligaments, and other periarticular structures without unduly interfering with the normal motion of the joint. Still more particularly, according to one aspect of the invention, a joint supporting device is provided that comes into play primarily as the horse fatigues, for example, towards the end of a race, when it is most vulnerable to damage. Several different and complementary ways in which this can be accomplished are disclosed herein. In other embodiments of the invention, a joint support device is provided intended primarily for rehabilitation after surgery or injury, where fatigue per se is less significant.
Referring now to the typical damage mechanisms experienced by horses, injuries resulting from accumulated micro-damage, which from a clinical perspective are equally if not more prevalent than injuries from a single traumatic event, have predilection sites, which are in turn linked to specific athletic activities. For example, the superficial digital flexor tendon (SDFT) in the mid to proximal metacarpal region of the front limb is the most frequently injured locus in racing thoroughbreds while the suspensory ligament (SL) is more frequently injured in racing standardbreds. Deep digital flexor tendon (DDFT) injury is most commonly encountered in jumping horses while hind limb proximal suspensory injury is more common in dressage horses. Similarly, bone and cartilage injury secondary to accumulated microdamage have predilection sites, for example, the proximal-dorsal aspect of the first phalanx within the fetlock joint. In each case, the likelihood of injury appears to increase with fetlock hyperextension, that is, extension of the joint beyond its normal range of motion.
Referring now specifically to the equine fetlock, within that region lie three particularly ‘at-risk’ principal soft tissue support structures (the SDFT, the DDFT and the SL) on the palmar/plantar (back) aspect of the bones, which work in unison with the limb's many other soft tissue components (e.g., the joint capsule, annular ligaments, and extensor tendons) to effect locomotion.
To best understand how the current invention will prevent injury to the SDFT, DDFT and SL, their anatomical and functional characteristics will be reviewed.
Collectively, the SDFT, DDFT and SL are substantially modified muscles, possessing short muscle fibers, a pennate structure (that is, comprising a muscle in which fibers extend obliquely from either side of a central tendon) and significant passive elastic properties. The SL, an evolutionary modification of the interosseus muscle, is completely fibrous with only remnants of muscle fibers to be found. The superficial digital flexor muscle (proximal to but contiguous with the SDFT) is also almost completely fibrous in the hind limb and in the forelimb has short muscle fibers of 2-6 mm length. These are primarily ‘slow’ muscle fibers best suited to supportive rather than propulsive functions by means of constant or extended length activity. The deep digital flexor muscle has three heads or muscle compartments (humeral, radial and ulnar) composed of varying numbers of short, intermediate, and long muscle fibers. It combines slow muscle fibers with a substantial population of ‘fast’ fibers which are better suited to propulsive functions. The SDFT and DDFT muscles are protected by accessory ligaments that link the tendon, distal to the muscle belly, to bone, effectively protecting the muscle and limiting the overall stretch (strain) that can be effected through the structure.
Once a horse has expended substantial energy in accelerating to a constant speed, a primary goal is to maintain that speed while minimizing the subsequent use of energy. The SDFT and DDFT have a major role in this process wherein their largely tendinous composition allows them to store and then return elastic energy, in the manner of a spring being stretched to store energy and then released to expend the stored energy. The SDFT and DDFT do this with remarkable efficiency, returning about 93% of the energy stored, much of the rest being dissipated as heat.
During the energy storage process at the time of weight-bearing, the tendons are substantially stretched. Under normal circumstances, the amount of stretching which they sustain (which may be as much as 8-12% of the resting length) remains within physiologically normal limits, allowing the tendons to recover their original form without injury. This elastic increase in length is the very means by which energy is efficiently stored in the same way that a spring stores energy by stretching, as above.
However, during extremes of activity the tendon or ligament can be stretched so much, particularly as the horse fatigues, that micro- and sometimes macro-damage occurs. Progressive degenerative changes within the tendon or ligament may precede and predispose to this injury. Given sufficient recovery time, micro-damage can often be repaired. If not, micro-damage can accumulate leading to macro-damage. The dividing line between the tendon strains (that is, the amount by which it is stretched) required to achieve efficient elastic energy storage and those which result in disruption of the tendon microstructure is very fine. If disruption does occur but is limited to a very small volume, the damage can be accommodated without compromising function, but when the injury is more widespread, clinical unsoundness can result.
At rest the SL is fully capable of passively resisting change in fetlock angle. At speed, however, the SDFT and DDFT provide additional support for the fetlock, countering the substantial weight-bearing forces, which tend to hyper-extend the joint. A controlled increase in joint extension is preferred. Towards extremes of exertion, however, the fast muscle fibers of the DDFT become fatigued and, with the passive SL limited in its ability to provide additional support as determined by its architecture, the SDFT is increasingly responsible for countering hyper-extension of the fetlock. Eventually, the SDFT can also be overloaded, the joint progresses to hyperextension, and damage ensues.
The SDFT and DDFT have additional roles, including the damping of the high-frequency (30-50 Hz) vibrations that occur at foot impact and which otherwise would cause increased onset of structural fatigue damage within bone and soft tissue, by increasing the number of loading cycles and the loading rate experienced by the limb.
Other comments regarding modes of injury include the following:
In addition to injury of the principle flexor soft-tissue structures (SL, SDFT and DDFT), fetlock hyperextension can also cause injury of hard tissue structures of the lower limb. For example, with fetlock hyperextension, the increasing forces exerted on the cartilage and underlying bone of the dorsal peripheral margin of the fetlock joint can cause microfracture. If given insufficient time to heal, accumulated microdamage eventually results in clinical injury caused, for example, by cartilage cracking and associated osteoarthritis or even bone fracture.
While the etiology (i.e., cause) of some SDFT, DDFT, and SL injuries are better understood than others, the final common pathway is one of mechanical disruption of collagen, the principal component of tendon and ligament, at a microscopic and sometimes macroscopic level. Concurrent with the disruption of the individual collagen fibers or bundles of fibers is local bleeding and resultant inflammation. Clinically this is characterized by pain, heat and swelling. The blood clot is subsequently resorbed and/or replaced by new collagen fibers laid down in a new extracellular matrix (bed), initially in random configuration. Finally, the collagen undergoes remodeling and is realigned to best offset the loading forces extant at that location. The process in its entirety takes up to one year to complete. As stated previously, the scar tissue thus formed is generally inferior in its mechanical qualities to uninjured tendon, predisposing the limb to reinjury.
Which of the support structures of the limb that is injured in any particular case, and where the injury might occur along its length, is predicated on multiple factors including but not limited to blood supply, pre-existing injury, degenerative disease, point of focal loading, activity type and quite possibly a series of mechanical parameters with pertinence to joint dynamics as well as the visco-elastic nature of tendons and ligaments.
It will be apparent that to the extent the fetlock joint can be prevented from being hyperextended, loading the support structures beyond their normal elastic limits, and possibly also causing hard tissues from experiencing excessive compression stress, injury to both soft and hard tissue can be limited. Intuitively, limiting extremes of fetlock motion would appear to be most easily achieved by physically restricting the upper limits of flexion and particularly extension in the longitudinal axis (forward and backward). Various types of boot and bandage have been studied with this goal in mind in previous reports. The data is often contradictory. Crawford et al. (1990a,b) found that different bandaging techniques and materials significantly influenced the energy absorption capacity of these bandages. Keegan et al. (1992) showed that support bandages did not alter mean strain in the suspensory ligaments while the horses were standing or walking. Using a tensile testing machine, Balch et al. (1998) demonstrated in an in vitro setting that certain types of support boots could absorb up to 26% of total force. However, in a similar set-up, Smith et al. (2002) found no difference between limbs with and without neoprene support boots. Kicker et al (2004) found some support boots to provide a significant reduction in total joint extension of up to 1.44 degrees at the trot, the practical implications of which have yet to be determined. Ramon et al (2007) found that athletic taping of the fetlock did not alter the kinematics of the forelimb during stance, but does limit flexion of the fetlock by approximately 5 degrees during the swing phase. A decreased peak vertical force also resulted, quite possibly due to an increased proprioceptive effect. Finally, Swanstrom (2005) shows soft tissue strain with fetlock angle for SDFT, DDFT and SL.
The present inventors estimate from this data that limiting the fetlock extension by 8 degrees is required to achieve a 10% reduction in extension of the SDFT, DDFT, and SL.
Similarly, it will be intuitively apparent that limiting the angular velocity of the joint—that is, the rate at which the joint is moved between extension and flexion—will be useful in preventing injury. More specifically, increasing the load rate on a visco-elastic material such as soft tissue increases the stiffness of the material, that is, increases its resistance to stretching. This in turn may increase the likelihood of the tissue tearing. Conversely, reducing the angular velocity implies that one is probably (though not definitely) reducing the load rate. With viscoelastic tissues, this will make them less stiff and thereby they should offer less resistance to load and hence they should experience less likelihood of tearing.
The objects of the invention are therefore to address the following biomechanical protection strategies. As will become apparent from the discussion of the several embodiments of the device of the invention described below, not all of the embodiments are directed toward each of these points.    1. Limitation of longitudinal or mediolateral ultimate joint flexion or extension, that is, limitation of the range of motion of the joint;    2. Limitation of longitudinal or mediolateral rate of joint flexion or extension, that is, limitation of the angular velocity of the joint;    3. Limitation of flexor apparatus ultimate load, that is, limitation of the loading experienced by the tendons and associated structure;    4. Limitation of flexor apparatus load rate, that is, limitation of the rate at which the tendons and associated structure are loaded;    5. Re-distribution of ground reaction forces away from bone to more superficial soft tissues.    6. Dissipation of ground impact-derived concussive forces.
Furthermore, it is important that these be accomplished without adversely affecting the horse's proprioceptive ability, while interfering with the horse's normal motion as minimally as possible, and while limiting overheating of the joint insofar as possible.